Add Chef's encrypted databag secret key.

Knowing that Chef offers the way to specify the location of
the encrypted databag secret key,
add Chef's encrypted databag secret key.

see: http://kataring.hatenablog.com/entry/2013/05/15/193430?utm_source=buffer&utm_medium=twitter&utm_campaign=Buffer&utm_content=buffer0817b

```
secret = Chef::EncryptedDataBagItem.load_secret("/tmp/chef-solo/data_bag_key/secret_key")
git = Chef::EncryptedDataBagItem.load("apps",  "git",  secret)
id_rsa = git["id_rsa"]
```

This commit fixes #200.
Merge branch 'id/200'